Evolution of mediated cooperative interactions

Melissa Whitaker, University of California, Davis, outlines the results of her YSSP project in which she modeled the effects of interaction asymmetries in order to explore the role of functional diversity on interaction dynamics.

M. Whitaker

M. Whitaker


Mutualisms—positive interactions among individuals of different species—are ubiquitous in nature and essential in the evolution and ecology of all life [1]. Previous empirical and theoretical studies of these interactions are severely limited by their typical casting of mutualisms as strictly pairwise interactions, despite the growing recognition that the vast majority of mutualisms involve multiple partners, often with the exchange of different services and important evolutionary tradeoffs. Attempts to understand this interaction complexity have typically considered assemblages of mutualists that provide functionally similar rewards and/or exploit the same resources in a similar way, such that even multispecies systems are reduced to functionally pairwise interactions. Thus, previous theoretical work may not capture the true dynamics of many mutualistic associations. Here, we include one additional player in models of interspecific mutualism and consider the effects of interaction asymmetries in order to explore the role of functional diversity on interaction dynamics.


Using a game theoretical approach, an individual-based model was constructed in which two concurrent pairwise games are played between a focal player and two lateral players. Players receive as payoff the sum of their own investments and the investments of the other players in the game, transformed with a saturating benefit function. This payoff structure approximates the ranking of payoffs in the classic Snowdrift game [2], but with continuous investments and independent benefit functions for each of the pairwise games. Players' investment strategies were compared under several biologically motivated scenarios: 1) symmetrical parameters for each of the pairwise games, 2) asymmetrical benefit functions, 3) asymmetrical allocation of the focal mutualist's investments, and 4) increasing the resources available to the focal mutualist.


In the 2-player game, one player acted cooperatively at intermediate levels while the other player became a parasite (these roles emerged stochastically). Interaction outcomes were similar in the 3-player symmetrical game; however, when asymmetries were introduced either in benefit functions or the focal mutualist's investments to lateral players, the outcomes became deterministic, and the identity of cooperative versus parasitic players was shown to be sensitive to model parameters.


Embracing the prevalence and complexity of multispecies mutualisms is predicted to have extensive influence on the field of biology. With the addition of a third player into a classic model we have demonstrated that interaction asymmetries can have important effects on interaction outcomes, suggesting that studies that overlook functional diversity (i.e., most previous theoretical work) are not applicable to many mutualisms. Given the ubiquity of mutualisms, including those among conservation targets (e.g., corals), economically important groups (e.g., pollinators), and ecologically dominant organisms (e.g., ants), such conceptual improvements can have broad influence on predictive ecology, ecosystem and resource management, and biodiversity conservation.


[1] Stachowicz, J. Mutualism, facilitation, and the structure of ecological communities. BioScience 51, (2001).

[2] Doebeli, M. & Hauert, C. Models of cooperation based on the Prisoner's Dilemma and the Snowdrift game. Ecology Letters 8, 748–766 (2005) [3] McFall-Ngai, M. Are biologists in “future shock”? Symbiosis integrates biology across domains. Nature reviews. Microbiology 6, 789–92 (2008).


Melissa Whitaker is a Chinese citizen, resident in the USA. She was funded by IIASA's United States National Member Organization and during YSSP worked in the Evolution and Ecology Program (EEP) with a special focus on system cascades and resource optimization.

Please note these Proceedings have received limited or no review from supervisors and IIASA program directors, and the views and results expressed therein do not necessarily represent IIASA, its National Member Organizations, or other organizations supporting the work.

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Last edited: 19 August 2015


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